The Software Development Life Cycle (SDLC) vs. The Software Testing Life Cycle (STLC)

Most technical teams are well-acquainted with the Software Development Life Cycle (SDLC)—a series of phases that guide a product from ideation to release. Every development methodology follows the SDLC in some form. But what many people don't realize is that running in parallel to the SDLC is the Software Testing Life Cycle (STLC). While both cycles share some similarities, the STLC has its own distinct set of phases, deliverables, and responsibilities, most of which fall under the purview of the lead QA.

In this post, we’ll explore the SDLC and STLC side by side, examining their differences, overlaps, and how they interact throughout the product lifecycle.

What is the SDLC?


The SDLC outlines the steps involved in building a software product. It provides a structured process for product teams to follow, ensuring that features are developed in an organized and systematic way. Here are the typical phases of the SDLC:

  1. Planning: Stakeholders define the overall idea, and high-level business requirements are set.
  2. Analysis: Market research and feasibility studies are conducted to determine if the feature is viable. This phase also involves a cost-vs-reward analysis and an estimation of timelines.
  3. Design: Product teams map out wireframes, and UI/UX teams begin designing the user interface and experience.
  4. Develop: Developers begin the actual coding and implementation of the feature.
  5. Testing and Integration: The end product is merged with existing features and deployed to a staging environment for QA testing.
  6. Maintenance: The feature is released to production, and the next iteration begins.

What is the STLC?


Parallel to the SDLC, the Software Testing Life Cycle (STLC) ensures that the software is properly tested and validated before it's released. The STLC also follows a structured set of phases, with each step managed by the lead QA to ensure thorough and efficient testing. Here are the key phases of the STLC:

  1. Requirements: Test requirements are derived from business requirements to ensure that each feature is testable.
  2. Test Plan: A document is created outlining the scope of testing, required resources, tools, environments, and test data. This is where test requirements are clearly defined.
  3. Test Design and Review: Test scenarios are created based on the test requirements. These scenarios are reviewed to ensure they align with the expected outcomes.
  4. Test Case Preparation: Test cases are created from the test scenarios. These are detailed, step-by-step instructions that will guide testers in executing the tests.
  5. Test Execution: This phase aligns with the Testing phase of the SDLC, where QA executes test cases on the product to verify its functionality.
  6. Test/Defect Report: During this phase, QA tracks defects and logs feedback, maintaining a detailed defect document to monitor progress and provide actionable insights.
  7. Retest Fixes/Regression Test: Any issues identified during testing are sent back to developers for fixes. Regression testing ensures that new changes don’t affect existing functionality.
  8. Software Release: Once testing is complete and issues are resolved, QA supports the deployment of the feature to production.

SDLC and STLC: Key Overlaps and Differences


At first glance, the SDLC and STLC might seem quite different, but there are significant overlaps, and they are closely intertwined. Let’s take a closer look at how the two cycles interact:

Planning and Requirements

  • SDLC (Planning): Stakeholders define the project scope, business goals, and high-level requirements.
  • STLC (Requirements): QA transforms business requirements into test requirements, ensuring everything that needs to be tested is clearly defined and measurable. This alignment is critical for developing an effective test strategy.

In many cases, Planning in the SDLC and Requirements in the STLC happen simultaneously. As business requirements take shape, QA teams begin mapping out their test approach.

Analysis and Test Plan

  • SDLC (Analysis): Market research and feasibility studies are conducted to assess the project's viability.
  • STLC (Test Plan): While the SDLC is analyzing feasibility, QA is busy creating a test plan that covers scope, resources, environments, and testing schedules. The Test Plan is continuously refined as more is learned about the project and requirements.

Design and Test Design

  • SDLC (Design): Designers start creating wireframes and mockups for the product's UI/UX.
  • STLC (Test Design): As wireframes take shape in the SDLC, QA begins designing test scenarios. In this phase, QA can also provide valuable feedback on design elements, helping ensure that the product is both functional and user-friendly. For example, QA might suggest design changes to improve usability or consistency across the application.

The Design phase of the SDLC and the Test Design phase of the STLC often run in parallel, with QA reviewing designs and beginning test case preparation.

Development and Test Case Preparation

  • SDLC (Development): Developers begin writing code for the feature.
  • STLC (Test Case Preparation): QA starts creating test cases based on test scenarios. At this stage, the test cases may include both functional and non-functional tests. QA teams may also begin working with developers to align on expectations for each feature.

Once development starts, test cases are created to guide testing, focusing on key functionality, edge cases, and potential issues.

Testing and Test Execution

  • SDLC (Testing and Integration): The software enters the testing phase, where it is integrated into the existing codebase and deployed to a staging environment.
  • STLC (Test Execution): During the SDLC testing phase, QA is executing test cases, tracking defects, and working with developers to resolve issues. This is the most intense phase for QA, and it involves both functional testing (verifying features) and regression testing (ensuring existing features haven’t broken).

This is the phase where both cycles overlap the most, as it is the primary phase for QA involvement in both the SDLC and STLC.

Release and Software Release

  • SDLC (Maintenance/Release): The feature is deployed to production, and the next iteration begins.
  • STLC (Software Release): QA supports the release to production, ensuring that everything is functioning as expected. After the release, QA teams perform post-deployment checks and monitor for any issues.

Final Thoughts: A Symbiotic Relationship

Though the SDLC and STLC have different phases and specific goals, they are closely intertwined. The STLC relies heavily on the successful and complete output of each SDLC phase to proceed effectively. While the SDLC focuses on building the product, the STLC ensures that product quality is maintained throughout the development process.

In Agile methodologies, these cycles may overlap or move at a faster pace, with iterations happening quickly. In Waterfall models, each phase may take longer, but the principle remains the same: the SDLC and STLC work hand in hand to deliver high-quality software.

By understanding both cycles and their interdependencies, teams can ensure better coordination, minimize defects, and ultimately deliver a successful product.

How does your team manage the SDLC and STLC? Do you see areas where the two cycles could be better integrated or streamlined? Share your thoughts or experiences in the comments below!

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